feat(backend): update Square integration, validators, and image validation

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/internal/images/validate_test.go

@@ -166,10 +166,10 @@ func TestValidateImageBytes_Empty(t *testing.T) {
 // TestValidateImageBytes_AllSupportedFormats verifies every supported format returns a non-empty extension.
 func TestValidateImageBytes_AllSupportedFormats(t *testing.T) {
 	tests := []struct {
-		name     string
-		data     []byte
-		wantExt  string
-		wantErr  bool
+		name    string
+		data    []byte
+		wantExt string
+		wantErr bool
 	}{
 		{"JPEG", []byte{0xff, 0xd8, 0xff, 0xe0, 0x00, 0x10, 0x4a, 0x46, 0x49, 0x46, 0x00, 0x01}, ".jpg", false},
 		{"PNG", []byte{0x89, 'P', 'N', 'G', '\r', '\n', 0x1a, '\n', 0, 0, 0, 0}, ".png", false},